[Rd] Undefined behavior of head() and tail() with n = 0

[Martin Maechler]

>>>>> Florent Angly <florent.angly at gmail.com>
>>>>>     on Wed, 25 Jan 2017 16:31:45 +0100 writes:

    > Hi all,
    > The documentation for head() and tail() describes the behavior of
    > these generic functions when n is strictly positive (n > 0) and
    > strictly negative (n < 0). How these functions work when given a zero
    > value is not defined.

    > Both GNU command-line utilities head and tail behave differently with +0 and -0:
    > http://man7.org/linux/man-pages/man1/head.1.html
    > http://man7.org/linux/man-pages/man1/tail.1.html

    > Since R supports signed zeros (1/+0 != 1/-0) 

whoa, whoa, .. slow down --  The above is misleading!

Rather read in  ?Arithmetic (*the* reference to consult for such issues),
where the 2nd part of the following section

 || Implementation limits:
 || 
 ||      [..............]
 || 
 ||      Another potential issue is signed zeroes: on IEC 60659 platforms
 ||      there are two zeroes with internal representations differing by
 ||      sign.  Where possible R treats them as the same, but for example
 ||      direct output from C code often does not do so and may output
 ||      ‘-0.0’ (and on Windows whether it does so or not depends on the
 ||      version of Windows).  One place in R where the difference might be
 ||      seen is in division by zero: ‘1/x’ is ‘Inf’ or ‘-Inf’ depending on
 ||      the sign of zero ‘x’.  Another place is ‘identical(0, -0, num.eq =
 ||      FALSE)’.

says the *contrary* ( __Where possible R treats them as the same__ ):
We do _not_ want to distinguish -0 and +0,
but there are cases where it is inavoidable

And there are good reasons (mathematics !!) for this.

I'm pretty sure that it would be quite a mistake to start
differentiating it here...  but of course we can continue
discussing here if you like.

Martin Maechler
ETH Zurich and R Core


    > and the R head() and tail() functions are modeled after
    > their GNU counterparts, I would expect the R functions to
    > distinguish between +0 and -0

    >> tail(1:5, n=0)
    > integer(0)
    >> tail(1:5, n=1)
    > [1] 5
    >> tail(1:5, n=2)
    > [1] 4 5

    >> tail(1:5, n=-2)
    > [1] 3 4 5
    >> tail(1:5, n=-1)
    > [1] 2 3 4 5
    >> tail(1:5, n=-0)
    > integer(0)  # expected 1:5

    >> head(1:5, n=0)
    > integer(0)
    >> head(1:5, n=1)
    > [1] 1
    >> head(1:5, n=2)
    > [1] 1 2

    >> head(1:5, n=-2)
    > [1] 1 2 3
    >> head(1:5, n=-1)
    > [1] 1 2 3 4
    >> head(1:5, n=-0)
    > integer(0)  # expected 1:5

    > For both head() and tail(), I expected 1:5 as output but got
    > integer(0). I obtained similar results using a data.frame and a
    > function as x argument.

    > An easy fix would be to explicitly state in the documentation what n =
    > 0 does, and that there is no practical difference between -0 and +0.
    > However, in my eyes, the better approach would be implement support
    > for -0 and document it. What do you think?

    > Best,

    > Florent


    > PS/ My sessionInfo() gives:
    > R version 3.3.2 (2016-10-31)
    > Platform: x86_64-w64-mingw32/x64 (64-bit)
    > Running under: Windows 7 x64 (build 7601) Service Pack 1

    > locale:
    > [1] LC_COLLATE=German_Switzerland.1252
    > LC_CTYPE=German_Switzerland.1252
    > LC_MONETARY=German_Switzerland.1252 LC_NUMERIC=C
    > LC_TIME=German_Switzerland.1252

    > attached base packages:
    > [1] stats     graphics  grDevices utils     datasets  methods   base

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